The National Zoo's Antarctica Expedition is sponsored by the National Science Foundation's Office of Polar Programs.

All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.

Class of 2007
March 11, 2008

The Antarctic summer is waning and our Weddell seal pups will soon be facing their first winter. How will they fare? How well did they cope with being left alone to fend for themselves?

A pup close to weaning looks for its mother. Photo by Regina Eisert/NZP

In December, when we last saw them, most of the pups were fat and sleek; many were crawling, and bawling after any adult female in search of mother, who had either left them forever or was planning to do so soon. Weaning is abrupt in most seals: In many species, a mother simply leaves her pups one day and never returns. In Weddell seals, the process is a little more gradual. A pup waits for its mother’s return from diving trips that last longer and longer until, finally, they are left alone.

In many cases, the female does not seem to move very far. Some of our animals moved from Hutton Cliffs to the nearby colony at Turtle Rock, a distance of only about two miles as the skua flies. But this is probably too far for young pups to swim on one breath under the closed sea-ice. At the age of six weeks, a Weddell seal pup can hold its breath for only a few minutes, whereas their elders may easily go without air for more than an hour or more.

Weddell seal pup underwater. Photo Regina Eisert/NZP

Weddell seal pups are unusual among seals in that when they are weaned, they have already been diving for three to four weeks, and spending several hours diving every day. So they are prepared to find their own food once their mothers leave. However, as we found out last year when we followed a group of pups for six weeks after weaning, four out of five pups lost a lot of weight during this time, suggesting that they are not very good at feeding themselves. As shown in Table 1, only one pup of five (6570) was even close to maintaining its weight, indicating that it had become a successful hunter.

PUP ID

Age 1 [days]

Age 2 [days]

Interval[days]

Mass loss [kg]

Loss[kg.d-1]

Loss[lbs.d-1]

6591

63

76

13

-8.5

-0.7

-1.4

6695

56

61

5

-2.5

-0.5

-1.1

6568

61

66

5

-2.5

-0.5

-1.1

6570

62

67

5

-0.5

-0.1

-0.2

6645

59

64

5

-4.0

-0.8

-1.8

There is reason to believe that pups that are too small or too lean when they are weaned do not survive the transition to independence. They simply run out of time to learn to support themselves before they burn up their body stores and starve. Thus, weaning weight and body condition of the pup appear to be very important to its survival, and we are trying to study how these parameters differ among mothers.

For example, do older or larger mothers have bigger, fatter pups? Do females that feed during lactation do better than those who do not? Do pups manage to catch prey before they are weaned? Do some mothers make more milk than others? Do all mothers produce milk of the same quality, or do some pups grow faster than others because they get better milk? And why?

So far, we have looked at how well pups grow during the first 12 days of life. Lactating females consume little or no food during this early period and pups have not yet started to dive, and so this is a good way to compare pup growth without the effects of additional variables, such as differences in food intake or activity.

First, there was no significant difference between pups in the first and the second season. The mean growth rate of pups was 2.1 kilograms (4.5 pounds) per day in 2006 and 2.0 kilograms (4.4 pounds) per day in 2007, or on average an amazing 6.5 percent per day.

Second, we looked at some of the factors we thought might influence pup growth. We had assumed that larger mothers would have larger pups that grow faster, but this is not really so. Preliminary analysis indicates that the size of the mother explains only a small proportion (20 percent) of the observed differences in pup growth rates. And the differences between pups are enormous, even at 12 days.

Egbert and his mother. Photo by Lisa Ware/NZP

Pup 7666, known to us as Egbert, or Eggie (Figure 5), grew more than 2.7 kilograms per day (6.0 pounds per day) and grew faster than all the other pups in two years of study. At 44 days of age, Egbert weighed an incredible 141 kilograms (312 pounds) and was presumably still growing because his mother was still around.

A very fat Weddell seal pup. Photo by Lisa Ware/NZP

Some pups become so fat, they have trouble moving (Figure 6). But what in humans would be called childhood obesity is a life-saver for these pups, a gift imparted to them by their mothers to help them survive the dangerous and solitary transition to full independence.